This invention relates to a chip-type solid electrolytic capacitor having an improved anode terminal.
In conventional chip-type solid electrolytic capacitors, the capacitor element is enclosed in an insulating resin cover layer. Usually the capacitor element uses a cylindrical (or prismatic) anode body made of a valve metal such as tantalum or aluminum, and a pin-like anode lead made of the same valve metal is planted into the anode body so as to protrude from one end face of the anode body. On the opposite end face and the cylindrical side face of the anode body there is a dielectric oxide layer formed by an anodization process, and the dielectric oxide layer is overlaid with a solid electrolyte layer, which is overlaid with a cathode conductor layer.
In rather primitive chip-type solid electrolytic capacitors, an external anode lead is soldered to the anode lead of the capacitor element and an external cathode lead to the cathode conductor layer of the element, and an insulating resin cover layer is formed by transfer molding so as to tightly enclose the entirety of the capacitor element except the extended end portions of the external anode and cathode leads. A disadvantage of this construction is the lowness of volumetric efficiency because of bulkiness of the resin cover layer in which the joints of the anode and cathode leads are embedded together with the capacitor element.
With the aim of reducing the overall size of a chip-type solid electrolytic capacitor and thereby raising volumetric efficiency, it has already been proposed to first cover the capacitor element with an insulating resin cover layer so as to leave the protruding anode lead and the cathode conduct layer on the opposite end face exposed and then form an anode terminal on the resin cover layer so as to make direct contact with the anode lead and a cathode terminal over the opposite end area so as to make direct contact with the exposed area of the cathode conductor layer. The anode terminal has a three-layer structure consisting of a conductive layer which lies on the surface of the resin cover layer and adheres to the protruding anode lead, a plating layer formed on the conductive layer and a solder layer on the plating layer. The conductive layer is formed by a thick-film process using a conductive paste. The overlying plating layer makes good adhesion to this conductive layer.
However, the anode terminal of the above described three-layer structure has a shortcoming. That is, the thermal expansion coefficient of the conductive layer (formed by using a conductive paste containing a binding resin) is larger than that of the anode lead (valve metal) by a factor of more than 10, so that there is a possibility of partial peeling of the conductive layer from the anode lead. Therefore, the electrical connection between the anode lead and the anode terminal is not sufficiently high in reliability, and there is a possibility of an increase in the dielectric loss of the capacitor.
To solve the above explained problem, there is a proposal of modifying the three-layer structure of the anode terminal to a two-layer structure by omitting the conductive layer. That is, in the modified anode terminal a plating layer makes direct contact with the anode lead, and the resin cover layer is overlaid with a solder layer. In this case there is little problem about the electrical connection between the anode Lead and the plating layer. However, the strength of adhesion of the anode terminal to the resin cover layer becomes weak since the plating layer is formed directly on a very smooth surface of the resin layer formed by molding. Therefore, when the chip-type capacitor is mounted on a printed circuit board by soldering the bonding strength of the capacitor chip is not sufficiently high.